Vehicular fuel level senders, as they are commonly called, are actually transducers that convert the level of fuel in the fuel tank into an electric signal to proportionally drive a visually readable electric instrument such as a bi-torque gauge. These fuel senders conventionally include a float carried on the end of a long pivoting arm suspended within the interior of the fuel supply tank, and this arm pivots a short wiper blade having a ball contact slideably engaging wires on a wound wire rheostat that varies in resistance in accordance with the position of the float and hence liquid level, however not always in directly proportional fashion. The angular position of the pivoting float arm, or more particularly the incremental angle of the arm, is not directly proportional to the vertical incremental change in float position and hence not directly proportional to liquid level. Moreover, in some cases, the instrument itself is not proportional throughout its range and requires some compensation.
Another cause for non-linearity is that fuel tanks frequently have non-symmetrical configurations because fuel tanks are frequently designed to fit into restricted envelopes within the vehicle. Irregular tank configurations thus introduce another non-linearity in sender readout and hence require compensation.
One way to compensate for these non-linearities is to vary the spacing between the wire turns on wound wire rheostats and more specifically by increasing wire turn spacing, the ratio of resistance change to float arm angle decreases, and conversely it increases with more tightly wound turns. Another common way of varying linearity is to change the individual turn lengths by modifying the shape of the support board on which the wire is wound. One common shape variation includes a tapered section in the form of a frusto-isosceles triangle.
This differential wire turn spacing as well as the wire turn length variation has created a problem because the wiper contact as it rides across the wire turns tends to pluck or "banjo" the loosely held wires and not infrequently causes rupture of the wire, which of course results in a complete failure in the rheostat.
The non-linearity produced by the pivoting float arm has been eliminated in some cases through the use of a vertically reciprocal float. The vertical float carries contact elements engageable with vertical straight wires, in one case, extending through the float so it produces incremental resistance changes directly proportional to incremental liquid level change and hence eliminates the inaccuracy produced by pivotal float arms. However, because the resistance elements in this design are wires, they have a uniform cross-section through their length and hence cannot compensate for the other non-linearities including fuel tank configuration. For this reason such fuel senders have not found widespread commercial success.
It is a primary object of the present invention to ameliorate the problems noted above in compensating for non-linearities in fuel sender.